Purification of aliphatic acids and anhydrides



Patented Apr. 7, .1942

{2.278331 5 seem cumsm Am sme ACID S' AND" summons 1 A m o.ceekernie, marsh-sine; va, Tv

' signoreto E. I. du'Pont dc Ncmours & Company 7 5 I wilminzton, .Del., a corporation of. Delaware I No Drawing. Application March 21,1941, 5

Serial No. 384,579

.8Claims. (!.260-1-541) ",j

invention relates tothe purification of aliphatic acid anhydrides and aliphatic acids.

More particularly it relates to'thetreatment of acetic anhydride' and acetic acid, to remove therefrom impurities which color and otherwise -It has been determined that the commercial that when cellulose is acetylated withpure acetic anhydridein the presence of pure aceticacid 1 and sulfuric acid as the catalyst,"the cellulose acetate product is muchimproved in color and clarity as compared with cellulonfacetate pro:

. duced with technical grade of acetic'anhydride f and/or technical grade of acetic acid. 6

While for some "uses 'a'hish degree of clari and the absence of color in the cellulose ester may be unnecessary, for the production of transparent film such as photographic film and-for 1 the production 'of transparent or lightly tinted" plastics, freedom from every trace of color and impurities effecting even a s'light-cloudingor col- 5 orlng ofcellulose acetate becomes important;

and it is therefore desirable that those impuri ties which lend color and deleteriouslyafiect claritybe removed fromthe anhydrlde and acid which are to be ter. 1

An object in the production of the es;

by a convenient and economical treatmenhf A.

still further object is to purify aliphatic acid an-'- hydrides. and aliphatic acidsihten'eralfand other objects inaiter.

v of-I-thisfinvention. is to provide; a method for efiectively removing from .acetici-an-f hydride and acetic acidflthose impurities which undesirably color and impair the clarityflof cer- .tain derivatives, particularly cellulose acetate.

A-further object is to substantially purifytech 1 nlcal grades of acetic anhydride and acetlcacid H -9: fconc. sulfuric acid,- it remained colorless forv-sev .yferalhoura- A will more clearly appear here- Z In. the preferred practice of the process the 7 amount 'of' yellowphosphorus is confined in-the range r about 0.025 to about 0.5% by *weigh't'o'f pho'spho'rusbased on the weight of the e deleteriously aflect the anhydride or .acid and products produced therefrom.

r or technical grades of acetic anhydride and aces tic acid contain impurities which impart'undesirable color to, and impair the clarity of cellulose acetate. Thisisdemonstrated by the fact .the treated anhydride (or acid) "Just" priortodistillatiomforinstancewith a'sodium salt of the aliphatic-acid corresponding to the anhydride ioracid) being puriiled'. v

f fill-reinvention is illustrated by th I "'e'xam'pleswhere'in are""by"wei8ht unless oihe is d-' x j time, had remained at about 90'? C. ior .two ,hours on the steamba h. 2 parts of anhydrous sodium These objects are accomplished by the-process 4! of this invention which, briefly stated, comprises adding a small quantity of yellow phosphorus to the aliphatic acid anhydride or aliphatic acid to be purified, then passing'air, oxygen, or gas containing oxygen in contactflwith= thephosphorus-containing mixture for a period of time while agitating and maintaining the same atan elevated temperature sufiicient to meltthe -phos I phorus; and thereafter; raplt lv distilling the an hydride or acid from the mixture.

' aliphatic acidanhydiide'or aliphatic acid.

The 'temperature ispreferably 1 maintained within the limits of from about C. to about we. in .7 i v Also, after the reaction has continued for the desired length ,oftime; it is preferable to neutralpart ofyellow phosphorus; The anhydride was heatedito (3., whereupon the phosphorus .meltedjand remained at the bottom of the flask. While, agitating the anhydride gently and -maina e p a u e. F1 -.'1;-1 was passed over the surface at the rateof 25 ml.

eilmmu ei After about .three hours; the phosphorus was all in'solution. .The agitation and pi f fie oiair were discontinuedfi After-the mixw we e fidild and the-mixtur distilled apidly 'with t mpt atfractionation: The

b mefl j e d t ete m r a s index of 0.6X10 andsav on a faint yellowish tinge when5. ml. werev treated slowlywith l ml.50% perchloricacidwithouteooling. When mixed slowly (cooling)-v withan equal volume of Thw'ne ne saelmae'xa sees a inthe for'egoing; @eqkample is a standard, mixer for in- 'dicating the purityofaceticanhydride'and acetic ac id..and is arrived at in accol'dancewithjthe 7 ji'ollowing procedure described in' connection with,v ll-acet i'flnhydride: 1 o

.siumpermanganate'per ewes volume of the v lution, usually'about 3.2grams per liter. When "following I T6 not ease: acetic anlsrdflhe,v having a. rmanganate index of 64x1'0 ,'was added 0.5

all measuredor weighed sample lof the anhydride is. to acetimacidby treatment with a large exce'ss'of dilute sulfurlceacid. The, dilute, acetic acid sulfuric acid solution obtained in thismanneris titrated with a solution containing aniexactly} k'nowulamountof potasanhydridein the tralilation.

read and the weight of permanganatecontained therein determined; 1 The permanganate-index" is then expressed as the number of grams-"of potassium permanganate consumedper titrated sample. Example II To 4000 parts of acetic anhydi'ide having a: permanganate index of 81x10- were added 4.

;-Example I. r

parts of yellow phosphorus. The anhydride was heated to 75-80 C. while air was passed in, in such a manner that the phosphorus was agitated continuously. After about four hours, all the phosphorus had either dissolved or changed to an orange-yellow powder which underwent no further change. Heating at about 90 iC.-was continued for three hours, after which 12 parts of anhydrous sodium acetate were added and the mixture subjected to distillation; The product had a permanganate indexof 0.4x10 andgave a yellowish tingewith one-fifth its volume of 50% perchloric. acidonly after several, hours;

with an equal volume of sulfuric acid, a faint discernible after sixteenhours.

orangev color was I 'Em'mpleIIf Two thousand (-2000) parts of acetic acid having a permanganate indexof 3x 10- and a freezing point of 155 C..wer e treated atom-'10" c. with 1 part of yellow phosphorus and air pre@ .viously dried over calcium chloride until the phosphorus had dissolved; Air. was then passed overthe surface at a rate for an addi-- tional two hours whileh'oldingthe temperature at 100 C. Anhydroussodium acetate sumcient to neutralize themineral acidity was then added and the acetic acid removed by distillation. The distilledacid had a permanganate index of less than 0.1x10- and a freezing point of -15.4 C. It showed no color when treated as above with perc'hloric and sulfuric acids.

Example IV 1 Eighteen hundred (1800) parts of acetic iicici having a permanganate index of 3x10 and a freezing point of 15.5 C. were mixed-with 200 parts of 98% acetic janhydride having a permanganate index of l22 10- One (llpart of used in this purification process is preferably' in the range from 0.025 to 0.5% basedon the- 0.5 part of yellow phosphorus,and the, heating period was omitted after thephosphorus had dissolved. The neutralization was effected by useof 3 parts of sodiumacetate. The distillate showed a permanganate indexof 0.5x 10- and gave somewhat less color with perchloric acid and sulfuric acid than the distillate obtained in While the process has been described in the above examples with specific reference to acetic anhydride and acetic acid, it is not so limited and may be applied as well in the purification of all aliphatic acid anhydrides and aliphatic acids such as propionic, butyric, and isobutyric acids and anhydrides. Likewise a wide variation from the exact amounts and conditions of the above examples may be practiced.

Thus, while the amount of yellow phosphorus weight of the acid or anhydride, some improvement will be obtained in the purity of the product by using asmailer amount than- .0.025%. Largeramounts than 0.5% maybe used, but there appears to be no advantage in using greater amount. v

The preferredtemperature range is from about C. to about 80? C. This is an optimum range in which the purification proceeds most advantageously. range, the phosphorus remainssolid and'ireacts slowly. 'At. higher temperatures (80; C.- to- 135 0.), the desired reaction still proceeds but the phosphorus dissolves less rapidly, presumably because of the lower solubility of oxygen in acetic anhydride at such temperatures Some improvement ,inthe purity of aliphatic acid anhydride or aliphatic acid can be obtained by this process in the range of temperatures from roomtemperature upto the boiling point of the acid or anhydride. At the lower temperature, the'phosphorus may be present in the form of fine granules so as to present a relatively large surface to the oxygen-containing medium. However, in view of the longer time required for reaction obviously it is preferred to heat the solution. 7 Instead of using yellow phosphorus the same i general effect is obtained by, the use of. equiv- 7 yellow phosphorus was added. The temperature'was raised to 0., afterwhich'theamixture was agitated while air waspassedover'the- After the phosphorus was (about three hours), the product wasdividaiifinto. 7 two equal parts. I To one half, 2 of acetate were added andthe mixture i distilledf surface.

The product contained 9.3% anhydrfide i perature of the treatment, the amountofsyellow phosphorusand the like, and may beidetermined bxgxinipleempirical tests in'each instance- ,The reaction product of phasphorusfl'with ima permanganate index of 0.4' 1 0 "it didnotjo color appreciably when treated with either'pery chloric acid or sulfuric acid as described in previous examples. without addition of sodium acetate. While thedistillate in this case was quite colorless, it had a permanganate index of 3x10 and was colored pale orange by one-fifth its volume of perchloric acid. Analysis showed 9.0% anhydride. Accordingly, it is clear that neutralization before distillation is the preferred procedure, though considerable purification is gained even without neu- Ezample V .ziConditions were as in Example 1, except that alent amounts of the lower oxides or acids of phosphorus; for instance, phosphorus ,trioxide. hypophosphorous acid and phosphorous acids may be without auxiliary oxygen.

The time required for the reaction will, of

"course, depend upona number of variables, in-

cluding the nature of the impurities, the temto "be entirelyvnon volatile under thefsimplest conditions of distillation. Since it The second half was distilled' 1. A'process for is known that'anhydride is not stable undericonditions of fractional-distillation, thebest procedure iors barating pure anhydride is to submit the treated anhydride to rapid distillation under a considerably reduced pressure'and with I claim:

purifying aliphatic acid anhydrides and aliphatic acids which comprises adding a relatively smail amount of yellow phosphorus to a relatively -'large amount of impure acidic material from the group consisting'of aliphatic acid anhydrides and aliphatic acids,

0.5 part of sulfuric acid was used in addition to contacting the mixture so formed with an oxy- 5. A process according to claim 1 wherein the gen-containing gas, and thereafter rapidly distilling said acidic material from said reaction mixture whereby to obtain the acidic material in substantially pure state.

2. A process for purifying aliphatic acid anhydrides and aliphatic acids which comprises adding a relatively small amount of yellow phosphorus to a relatively large amount of impure acidic material from the group consisting of aliphatic acid anhydrides and aliphatic acids; contacting the mixture so formed with an oxygen-containing gas while maintaining the temperature of said mixture within the range of from about 45 C. to about 80 C., and thereafter rapidly distilling said acidic material from said reaction mixture whereby to obtain the acidic material'in substantially pure state. 7

-3. A process for purifying aliphatic acid anhydrides and aliphatic acids which comprises adding a relatively small amount of yellow phosphorus to a relatively large amount of impure acidic material from the group consisting of aliphatic acid anhydrides and aliphatic acids, contacting the mixture so formed with an oxygen-containing gas 1. while maintaining the temperature of said mixture within the range of from about 45 C to about 80 C., neutralizing the resulting reaction mixture with in inorganic salt of the acidic material, and thereafter rapidly distilling said acidic material from said reaction 'mixture whereby to obtain the acidic material in substantially pure state.

4. A process according to claim 1 wherein the acidic material is acetic anhydride.

acidic material is acetic acid. I g

6. A process according to claim 3 wherein the amount of yellow phosphorus added is withinthc range of from about 0.025% to about 0.5% by weight based on the weight of the acidic material.

"l. The process which comprises adding to impure acetic anhydride from about 0.025% to about 0.5%by weight based on the weight of acetic anhydride, of yellow phosphorus, passing air in intimate contact with theresulting mixture, while maintaining said mixture at a temperature within the range of from about 45 C. to about 80 C., neutralizing the resulting reaction-mixture with anhydrous alkali-metal acetate-and thereafter rapidly, distilling acetic anhydride from said reaction mixturewhereby to obtain acetic anhydride' substantially free of impurities.- a

8 The process-which comprises adding to impure acetic acid. from about 0.025% to about 0.5% by weight based on the weight of acetic acid, of yellow phosphorus, passing air'in intimate contact with the resulting mixture, while maintaining said mixture at a temperature within the range of from about 45 C. to about 80 C., neutralizing the resulting reaction mixture with alkali-metal acetate, and thereafter rapidly distilling acetic acid from said reaction mix ture whereby to obtain acetic acid substantially free of impurities.- I 

